CENG 594 – Environmental Engineering Lecture Note
Chapter 5 Air Pollution and Control
5.1 Introduction
In addition to global warming and stratospheric ozone depletion, air pollutants also pose local
and regional hazards.
Air pollution is the presence in the outdoor atmosphere of one or more air contaminants (i.e.,
dust, fumes, gas, mist, odor, smoke, or vapor) in sufficient quantities, of such characteristics, and
of such duration as to be or to threaten to be injurious to human, plant, or animal life or to
property, or which reasonably interferes with the comfortable enjoyment of life or property.
Human activities are the main causes of air pollution problems that threaten to make portions of
the earth’s atmosphere an inhospitable environment.
In some cases we do not control emissions adequately. We must then rely on dispersion and
subsequent natural atmospheric cleansing processes to avoid exceeding pollutant concentrations,
which would result in undesirable effects.
Throughout the world, emphasis has been given on controlling the ambient atmospheric
concentrations of pollutants to levels at which no health effects will be observed. Control of air
pollution is not always easy, for it is impractical to eliminate all emissions of a specific pollutant.
On the other hand, it is reasonable to expect control of emissions to the lowest possible level
consistent with available technology and within reasonable cost. In practice, emission limits or
standards are frequently established rather than ambient air quality standards, because they are
far easier for control agency to enforce, although it is the latter that are really desired.
Composition and Structure of the Atmosphere
The total mass of each gas in the atmosphere is given in table 5-1. Varying amounts of most of
these gases may be found in each of the four major layers of the atmosphere—the troposphere,
the stratosphere, the mesosphere, and the thermosphere (see Fig. 5-1).
The layer of greatest interest in pollution control is the troposphere, since this is the layer in
which most living things exist. One of the most recent changes in the atmosphere involves the
phenomenon of acid rain. The amount of carbon dioxide is reported to be increasing at a rate of
1.8 mg/m3 per year, a process that may not be reversible. Furthermore, this increase has been
accompanied by an equivalent decrease in the atmospheric oxygen (O2).
5.2 Physical and chemical Fundamentals
Ideal Gas Law:
PV = nRT (5-1)
where P = absolute pressure, kPa
V = the volume occupied by n moles of gas
R = universal gas constant = 8.3143 J/K.mole
T = absolute temperature, K
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AAU, FoT, Department of Civil Engineering Instructor: Zerihun Alemayehu
CENG 594 – Environmental Engineering Lecture Note
Table 5-1. Concentration of atmospheric gases in clean dry air at ground level
Concentration, Concentration,
Gas
ppm by volume % by volume
Nitrogen 280,000 78.09
Oxygen 209,000 20.95
Argon 9,300 0.93
Carbon dioxide 320 0.032
Neon 18 0.0018
Helium 5.2 0.00052
Methane 1.5 0.00015
Krypton 1.0 0.0001
Hydrogen 0.5 0.00005
Dinitrogen Oxide 0.2 0.00002
Carbon monoxide 0.1 0.00001
Ozone 0.08 0.000008
Ammonia 0.006 0.000006
Nitrogen dioxide 0.001 0.000001
Sulfur dioxide 0.0002 0.0000006
Hydrogen sulfide 0.0002 0.0000002
Figure 5-1 Profile of the atmosphere
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AAU, FoT, Department of Civil Engineering Instructor: Zerihun Alemayehu
CENG 594 – Environmental Engineering Lecture Note
Dalton’s Law of Partial Pressure
The total pressure exerted by a mixture of gases is equal to the sum of the pressures that each
type of gas would exert if it alone occupied the container.
Pt = P1 + P2 + P3 + ….. (5-2)
where Pt = total pressure of mixture
P1, P2, P3 = pressure of each gas if it were in container alone, that is partial pressure.
Adiabatic Expansion and Compression
An adiabatic process is one that takes place with no addition or removal of heat and with
sufficient slowness, so that the gas can be considered to be in equilibrium at all times.
With the first principle of thermodynamics we have:
(Heat added to gas) = (increase in thermal energy) + external work done by or on the gas)
Since the left side of the equation is zero (because it is an adiabatic process), the increase in
thermal energy is equal to the work done. The increase in thermal energy is reflected by an
increase in the temperature of the gas. If the gas is expanded adiabatically, its temperature will
decrease.
Units of Measure
The three basic units of measure used in reporting air pollution data are micrograms per cubic
meter (µg/m3), parts per million (ppm), and the micron (µ) or, preferably, its equivalent, the
micrometer (µm). Micrograms per cubic meter and parts per million are measures of
concentration. Both µg/m3 and ppm are used to indicate the concentration of gaseous pollutant.
However, the concentration of particulate matter may be reported only as µg/m3. The µm is used
to report particle size.
Converting µg/m3 ppm. We can use the following formula to convert ppm to µg/m3.
10
/
/
This conversion is based on the fact that at standard conditions (0oC and 101.325 kPa), one mole
of an ideal gas occupies 22.414 L. Thus, we can convert the mass of the pollutant Mp in grams to
its equivalent volume Vp in liters at standard temperature and pressure (STP):
22.414 / (5—3)
where GMW is the gram molecular weight of the pollutant. For readings made at temperatures
(T2) and pressures (P2) other than standard conditions, the standard volume, 22.414 L/GM, must
be corrected using the ideal gas law. The following can be used the formula:
101.325
22.414 5 4
273
Since ppm is a volume ratio, we may write
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AAU, FoT, Department of Civil Engineering Instructor: Zerihun Alemayehu
CENG 594 – Environmental Engineering Lecture Note
5 5
where Va is the volume of air in cubic meters at the temperature and pressure of the reading.
Example 5-1. A one-cubic-meter sample of air was found to contain 80 µg/m3 of SO2. The
temperature and pressure were 25oC and 103.193 kPa when the air sample was taken. What was
the SO2 concentration in ppm?
53 Sources and Classification of Pollutants
To have an air pollution incident, or to have a problem, there are three factors that must occur
simultaneously. There must be sources, a means of transport, and receptors. Figure 5-2 illustrates
the process. Air pollution sources are relatively common knowledge. Their strength, type, and
location are important factors. By transport, reference is made to the meteorological conditions,
and the topography and climatology of a region, which are the important factors in dispersion —
that is, in getting the material from the sources to the receptors. The receptors include human
beings, other animals, materials, and plants. We also know that air pollution can affect visibility
and can endanger our lives simply by making it difficult to travel on the highways and difficult
for planes to land.
Figure 5-2. The trilogy: Sources — Transport — Receptors.
Air pollutant sources can be categorized according to the type of source, their number and spatial
distribution, and the type of emissions. Categorization by type includes natural and manmade
sources. Natural air pollutant sources include plant pollens, wind-blown dust, volcanic eruptions,
and lightning-generated forest fires. Manmade sources include transportation vehicles, industrial
processes, power plants, municipal incinerators, and others.
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AAU, FoT, Department of Civil Engineering Instructor: Zerihun Alemayehu
CENG 594 – Environmental Engineering Lecture Note
Source categorization according to number and spatial distribution includes single or point
sources (stationary), area or multiple sources (stationary or mobile), and line sources (see Fig 5-
3).
Figure 5-3. Source categories for emission inventories.
Classification of Pollutants
Air pollutants can be classified according to origin, chemical composition, and state of matter.
Origin. According to their origin, pollutants are considered as either primary or secondary air
pollutants. Primary air pollutants are pollutants in the atmosphere that exist in the same form as
in source emissions. Examples of primary air pollutants include carbon monoxide, sulfur
dioxide, and total suspended particulates. Secondary air pollutants are pollutants formed in the
atmosphere as a result of reactions such as hydrolysis, oxidation, and photochemical oxidation.
Secondary air pollutants include acidic mists and photochemical oxidants. In terms of air quality
management, the main strategies are directed toward source control of primary air pollutants.
The most effective means of controlling secondary air pollutants is to achieve source control of
the primary air pollutant; primary pollutants react in the atmosphere to form secondary
pollutants.
Chemical Composition. Air pollutants can further be classified according to their chemical
composition, as either organic or inorganic. Organic compounds contain carbon and hydrogen,
and many also contain elements such as oxygen, nitrogen, phosphorus, and sulfur. Hydrocarbons
are organic compounds containing only carbon and hydrogen. Inorganic materials found in
contaminated atmosphere include carbon monoxide (CO), carbon dioxide (CO2), carbonates,
sulfur oxides, nitrogen oxides, ozone, hydrogen fluoride, and hydrogen chloride.
State of Matter. As seen in Table 5-2, air pollution sources can also be categorized according to
whether the emissions are gaseous or particulates. Particulate pollutants, finely divided solids
and liquids, include smoke and dust emissions from a variety of sources. Examples of gaseous
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AAU, FoT, Department of Civil Engineering Instructor: Zerihun Alemayehu
CENG 594 – Environmental Engineering Lecture Note
pollutant emissions include carbon monoxide, hydrocarbons, sulfur dioxide, and nitrogen oxides.
Often, an air pollution source emits both gases and particulates into the ambient air.
Table 5-2 Classification of pollutants
Major classes Subclasses Typical members of subclasses
Particulates Solid Dust, smoke, fumes, fly ash
Liquid Mist, spray
Gases
Organic Hydrocarbons Hexane, benzene, ethylene,
methane, butane, butadiene
Aldehydes and ketones Formaldehyde, acetone
Other organics Chlorinated hydrocarbons, alcohols
Inorganic Oxides of carbon Carbon monoxide, carbon dioxide
Oxides of sulfur Sulfur dioxide, sulfur trioxide
Oxides of nitrogen Nitrogen dioxide, nitric oxide
Other inorganics Hydrogen sulfide, hydrogen fluoride,
ammonia
Criteria Air Pollutants
Two kinds of ambient pollutants are regulated under the Clean Air Act: Criteria pollutants and
hazardous air pollutants. The Clean Air Act characterizes five primary pollutants and one
secondary pollutant as criteria air pollutants. These six pollutants are emitted in relatively large
quantities by various sources and tend to threaten human health or welfare.
The five primary criteria pollutants include the gases sulfur dioxide (SO2), nitrogen oxides
(NOx), and carbon monoxide (CO) and solid or liquid particulates (smaller than 10 µm, PM-10)
and particulate lead. Ozone (O3) is the secondary criteria pollutant regulated under Clean Air
Act.
Carbon Monoxide
Nitrogen Dioxide
Sulfur Oxides
Effects of Particulates
Effects on human health – respiratory illness (depends upon the concentration)
Lead particulates – blood forming systems, the nervous system, and the renal system.
Effects on plants and animals – damage and inhibit growth to plant tissue
interfere with photosynthesis
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AAU, FoT, Department of Civil Engineering Instructor: Zerihun Alemayehu
CENG 594 – Environmental Engineering Lecture Note
Effects on materials – soiling clothing, corroding metals (RH > 75%), eroding building surfaces,
and discoloring and destroying painted surfaces.
Effects of Sulfur Oxides
Effects on human health – irritate the mucous membrane Æ bronchitis and pulmonary
emphysema
Effects on plants – damaging tissues, or bleaching
Effects on materials – sulfuric acid aerosols attack building materials Æ marble, limestone,
roofing slate, and mortar.
Effects of Nitrogen Oxides
Effects on human health – irritates the aveoli of the lung
high burning of eye
destroy the lining of the lung
lead to cancer
Effects of Carbon Monoxide
At high concentration (>2% COHb level) affects human aerobic metabolism
Affinity to Hb 200 times higher that O2
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AAU, FoT, Department of Civil Engineering Instructor: Zerihun Alemayehu
